Quantification of Free Sialic Acid in Urine by HPLC–Electrospray Tandem Mass Spectrometry: A Tool for the Diagnosis of Sialic Acid Storage Disease

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Abstract
Background: Sialic acid storage diseases (SSDs) are severe autosomal recessive neurodegenerative disorders caused by a transport defect across the lysosomal membrane, which leads to accumulation of sialic acid in tissues, fibroblasts, and urine. Defective free sialic acid transport can be established by quantification of free sialic acid in urine. Methods: Urine sample size was adjusted to the equivalent of 100 nmol of creatinine. After addition of 2-keto-3-deoxy-d-glycero-d-galactonononic acid as internal standard, samples were diluted with water to an end volume of 250 μL. We used 10 μL for HPLC–tandem mass spectrometric analysis in the negative electrospray ionization mode, monitoring transitions m/z 308.3→m/z 86.9 (sialic acid) and m/z 267.2→m/z 86.9 (internal standard). The overall method was validated and studied for ion suppression, interfering compounds, and pH effects. Samples from controls (n = 72) and SSD patients (n = 3) were analyzed. Results: The limit of detection was 3 μmol/L. Intraassay imprecision (CV; n = 10) was 6%, 3%, and 2% at 30, 130, and 1000 mmol/mol creatinine, respectively; corresponding interassay CV (n = 10) were 5%, 5%, and 2%. Recovery was 109% (100–1000 mmol/mol creatinine). The mean (SD) [range] excretion rates (mmol/mol creatinine) were 31.3 (16.6) [0.7–56.9] at 0–1 year (n = 20), 21.2 (9.8) [6.3–38.3] at 1–3 years (n = 15), 14.4 (8.2) [1.7–32.9] at 3–10 years (n = 25), and 4.6 (2.6) [0–9.8] above age 10 years (n = 12). SSD patients 1.2, 3.9, and 12 years of age had concentrations of 111.5, 54.2, and 36.1 mmol/mol creatinine, respectively. Conclusions: The HPLC-tandem MS method for free sialic acid in urine is more rapid, accurate, sensitive, selective, and robust than earlier methods and may serve as a candidate reference method for free sialic acid in diagnosis of SSD.